The use of chiral liquid crystals to give coloured films that exhibit an angular colour dependence has been described in prior art. Typically, these films are prepared from a polymerisable cholesteric liquid crystal mixture that is coated onto a substrate and polymerised. The nature of these films is that they exhibit a helically twisted structure and reflect circular polarised light of a specific waveband. The reflected light has a wavelength that depends on the helical pitch and a “handedness” of polarisation that depends on the twist sense of the helix structure of the liquid crystal film.
The chiral liquid crystal materials used for the preparation of coloured films usually comprise a liquid crystal host mixture and one or more chiral dopants that induce a helical twist with a given twist sense in the host mixture. The pitch p of the molecular helix in the first approximation, which is sufficient for most practical applications, is inversely proportional to the concentration c of the chiral dopant in the liquid crystal host mixture according to equation (1):                     p        =                              1            HTP                    ·                      1            c                                              (        1        )            
The proportionality factor is the helical twisting power (HTP) of the chiral dopant.
It is possible using the above described technology to undertake a two stage process and print or coat two different liquid crystal mixtures that differ only in the reflection wavelength of the reflected light, to form patterned polymer films with areas of different reflection colour. These films are useful as optical elements or for decorative or security applications, where they can be used as security markings exhibiting a visible pattern that is difficult to counterfeit.
However, the use of different mixtures and two coating steps complicates the manufacture of such films.
Another method to achieve patterned films is by using polymerisable chiral mixtures comprising a chiral compound that changes its structure upon irradiation, bringing about a change in the HTP of the compound and thus a change of the helical pitch and the reflection colour of the chiral mixture. In combination with a two-step irradiation and polymerisation process and conventional photomask techniques a patterned film with areas of different reflection colour can be formed. This is described for example in WO 00/034808. Furthermore, U.S. Pat. No. 5,668,614 describes the use of photoisomerisable compounds in multicolour cholesteric displays having regions of different reflection wavelength, but does not relate to patterned polymer films.
However, some applications especially in the security sector require films that exhibit a hidden pattern or image which can only be recognized by special means, e.g., when viewed through a polariser.
One aim of the present invention is to provide a polymer film with a hidden pattern that is stable against outside influences like heat, chemical solvents or mechanical stress, can be easily manufactured, and is suitable in particular as a false-proof security marking. Another aim is to provide methods for preparing such a film.
Upon further study of the specification and appended claims, further aims and advantages of this invention will become apparent to those skilled in the art.
These aims can be achieved by providing an anisotropic polymer film comprising a polymerised chiral liquid crystal material with helically twisted structure, wherein the helix axis is oriented substantially perpendicular to the plane of the film, and reflecting circularly polarised light, wherein the film further exhibits a pattern of different areas with different twist sense, leading to different polarisation state (“handedness”) of the reflected light. The chiral liquid crystal material can be selected such that the areas with different handedness have the same reflection colour, so that the pattern is not visible to the naked eye, but is visible only when viewed through a circular polariser.
WO 98/57223 discloses the use of chiral photoisomerisable compounds for preparing a multi-domain liquid crystal display having different areas with different twist sense (handedness). However, it does not mention coated polymer films.